Variable transmission devices are known. In one example type of such variable transmission devices, planetary members are provided in rolling contact with inner and outer races. The inner and outer races have two parts, the axial separation of which is adjustable. The transmission ratio of input to output speed of the device is adjusted by varying the axial separation of the outer race parts which causes a corresponding radial shift of the planetary members. The axial separation of the inner race parts adjusts to compensate for the radial change in position of the planetary members, as will be discussed in more detail below. The changing position of the contact points between the planetary members and the inner and outer races causes the change in transmission ratio of the device. Typically, the inner race is coupled with an input shaft and the planetary members are coupled via a planet follower arrangement to an output shaft. To enable the axial separation of the inner race parts to adjust, a helical screw coupling is provided on the input shaft so that rotation of the input shaft in the intended direction causes the two parts of the inner race to approach each other until the force exerted on the helical interengagement between the inner race and the input shaft matches the reaction forces between the inner race and the planetary members. When these forces match, no further relative axial displacement of the inner race parts takes place and a drive torque is transmitted at the transmission ratio determined by the radial position of the planetary members when this occurs.
Although this arrangement enables drive torque to be transmitted between the input and output shaft at the appropriate transmission ratio, undesirable consequences can occur.
Accordingly, it is desired to provide an improved variable transmission device.